With the increasing demand on capacity and system flexibility, optical coherent technology has again attracted more-and more attention. In comparison with incoherent technology, such as direct detection of On-Off keying (OOK), and delayed detection, namely self-coherent technology of differential phase shift keying (DPSK), the coherent technology has the following advantages: it has approximately 3 dB optical signal to noise ratio (OSNR) benefit as compared with differential detection; it can use equalization technology to countermeasure linear distortion in optical domain; and it can deal with advanced modulation schemes such as quadrature amplitude modulation (QAM).
A very important issue in a coherent receiver is how to recover the carrier phase. This can be realized by feed back control (phase locked loop), or by feed forward phase estimation. Both methods require the frequency offset between the carrier and the local oscillator to be small enough. This is usually satisfied by device specification, or by automatic frequency control (AFC). Francis D. Natali summarizes several AFC methods in “AFC tracking algorithms”, IEEE transactions on communications, Vol. Com-32, No. 8 August 1984, P935. The typical analog method is differentiator AFC which requires an analog differentiator. It is common in electrical communication, but such device is very difficult for implementation in the optical communication system, since the bandwidth of an electric signal in optical communication is in the scale of ˜10 GHz. The typical digital method is the cross product AFC, but it requires the digital signal processing (DSP) capability at the speed of ADC sampling rate. The sampling rate in optical communication is also in the scale of ˜10 GHz, for example 40 G samples/second. Such a high speed DSP is currently very difficult.
FIG. 1 shows an optical coherent receiver that makes use of cross product automatic frequency control of the prior art. The optical coherent receiver comprises a local oscillator 103, for supplying a local oscillator optical signal; an optical hybrid 102, for mixing an optical input 101 with the local oscillator optical signal; a first twin photoelectric detector 104 and a second twin photoelectric detector 105, for converting the optical signals outputted from the optical hybrid into baseband electric signals; an automatic frequency control section, for monitoring frequency offset of the optical input 101 in accordance with the converted signal, and controlling the local oscillator 103 to output the local oscillator optical signal in accordance with the frequency offset; and a signal processing section 106, for processing the converted signal to obtain a data output. The optical coherent receiver receives an optical input, converts the optical input into a baseband electric signal including a cophase component I and a quadrature component Q, and outputs the signal to the signal processing section 106, which processes the inputted signal to obtain the data output.
The automatic frequency control section comprises a control logic 111 and a frequency offset monitor 109, of which the control logic 111 controls the frequency of the local oscillator optical signal of the local oscillator 103 in accordance with the frequency offset.
The first input terminal of the optical hybrid 102 is connected to the optical input 101, the second input terminal thereof is connected to the output terminal of the local oscillator 103, the first and second output terminals of the optical hybrid 102 are respectively connected to the first and second input terminals of the first twin photoelectric detector 104, the third and fourth output terminals thereof are respectively connected to the first and second input terminals of the second twin photoelectric detector 105, the output terminals of the first twin photoelectric detector 104 and the second twin photoelectric detector 105 are respectively connected to the first and second input terminals of the signal processing section 106, and the output terminal of the first twin photoelectric detector 104 is further connected to the first input terminal and the second input terminal of a frequency offset monitor 109, the output terminal of the second twin photoelectric detector 105 is further connected to the third input terminal and the fourth input terminal of the frequency offset monitor 109, the input terminal of the control logic 111 is connected to the output terminal 118 of the frequency offset monitor 109, and the output terminal of the control logic 111 is connected to the input terminal of the local oscillator 103.
The frequency offset monitor 109 comprises: a first delayer 114, for delaying a fourth input signal inputted via the fourth input terminal of the frequency offset monitor 109 by a predetermined time, and outputting the delayed fourth input signal; a second delayer 115, for delaying a second input signal inputted via the second input terminal of the frequency offset monitor 109 by a predetermined time, and outputting the delayed second input signal; a first multiplier 112, for calculating a product of a first input signal inputted via the first input terminal of the frequency offset monitor 109 and the delayed fourth input signal, and outputting the calculation result; a second multiplier 113, for calculating a product of a third input signal inputted via the third input terminal of the frequency offset monitor 109 and the delayed second input signal, and outputting the calculation result; a subtracter 116, for calculating a difference obtained by subtracting the calculation result of the first multiplier 112 from the calculation result of the second multiplier 113, and outputting the difference; and an averager 110, for calculating an average value of the difference outputted by the subtracter 116 as the result of frequency offset monitoring. The input terminal of the first delayer 114 is connected to the fourth input terminal of the frequency offset monitor 109, the output terminal of the first delayer 114 is connected to the second input terminal of the first multiplier 112, the input terminal of the second delayer 115 is connected to the second input terminal of the frequency offset monitor 109, the output terminal of the second delayer 115 is connected to the second input terminal of the second multiplier 113, the first input terminal of the first multiplier 112 is connected to the first input terminal of the frequency offset monitor 109, the output terminal of the first multiplier 112 is connected to the negative input terminal of the subtracter 116, the first input terminal of the second multiplier 113 is connected to the third input terminal of the frequency offset monitor 109, the output terminal of the second multiplier 113 is connected to the positive input terminal of the subtracter 116, the output terminal of the subtracter 116 is connected to the input terminal of the averager 110, and the output terminal of the averager 110 is connected to the output terminal 118 of the frequency offset monitor 109.
The aforementioned device feeds the received optical input 101 (such as s(t)exp(jωt), where s(t) is modulation baseband data signal, and ω is carrier frequency) and the local oscillator optical signal (such as exp(jωLt), where ωL is frequency of the local oscillator) of the local oscillator 103 into the optical hybrid 102. The optical hybrid 102 generates a first, a second, a third, and a fourth optical signals: S+L, S−L and S+jL, S−jL, where S is the received optical input 101, L is the local oscillator optical signal. The first twin photoelectric detector 104 and the second twin photoelectric detector 105 convert the four optical signals to two baseband electric signals. The two electric baseband signals are respectively a cophase signal I 107 and a quadrature signal Q 108, and there exists I+jQ=s(t)exp(jΔωt), where Δω=ω−ωL is frequency offset between the local oscillator and the carrier.
The signal processing section 106 recovers data from the cophase signal I and the quadrature signal Q by performing analog or digital carrier phase recover, match filter or data recover, etc. This can be realized in the analog domain or the digital domain. The frequency offset monitor 109 is a self-correlation calculator, whose output 118 is:
                                                                        〈                                                                            -                                              I                        ⁡                                                  (                          t                          )                                                                                      ⁢                                          Q                      ⁡                                              (                                                  t                          -                          τ                                                )                                                                              +                                                            Q                      ⁡                                              (                        t                        )                                                              ⁢                                          I                      ⁡                                              (                                                  t                          -                          τ                                                )                                                                                            〉                            =                            ⁢                              Im                (                                  〈                                                            s                      ⁡                                              (                        t                        )                                                              ⁢                                          exp                      ⁡                                              (                                                  jΔ                          ⁢                                                                                                          ⁢                          ω                          ⁢                                                                                                          ⁢                          t                                                )                                                              ⁢                                                                  s                        *                                            ⁡                                              (                                                  t                          -                          τ                                                )                                                                                                                                                                                                    ⁢                                  exp                  ⁢                                      (                                                                  -                        j                                            ⁢                                                                                          ⁢                      Δω                      ⁢                                                                                          ⁢                      t                                        )                                    ⁢                                      exp                    ⁡                                          (                                              j                        ⁢                                                                                                  ⁢                        Δω                        ⁢                                                                                                  ⁢                        t                                            )                                                                      〉                            )                                                                          =                            ⁢                              Im                ⁢                                  (                                                            exp                      ⁡                                              (                                                  jΔ                          ⁢                                                                                                          ⁢                          ω                          ⁢                                                                                                          ⁢                          t                                                )                                                              ⁢                                          〈                                                                        s                          ⁡                                                      (                            t                            )                                                                          ⁢                                                                              s                            *                                                    ⁡                                                      (                                                          t                              -                              τ                                                        )                                                                                              〉                                                        )                                                                                                        =                            ⁢                                                sin                  ⁡                                      (                                          Δ                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        )                                                  ⁢                                  〈                                      s                    ⁢                                          (                      t                      )                                        ⁢                                                                  s                        *                                            ⁡                                              (                                                  t                          -                          τ                                                )                                                                              〉                                                                                        (        1        )            
In Equation 1, τ corresponds to the delay of the delayer 114/115, “<.>” indicates averaging, and the upper mark * indicates conjugation of complex numbers. The output 118 is proportional to the frequency offset, if the self-correlation of signal s(t) is a real number. The coefficient of the proportion is not zero as long as the delay is shorter than the correlation of the modulated signal s(t). The control logic 111 and the local oscillator 103 are the same as those conventionally used in a coherent receiver.
In order to correctly realize Equation (1), the speed of the first multiplier 112, the second multiplier 113, the first delayer 114, the second delayer 115 and the subtracter 116 should match the bandwidth of s(t) in either the digital domain or the analog domain. The bandwidth of s(t) in optical communication is usually in the scale of ˜10 GHz. Such ˜10 GHz high speed device is difficult for the current electrical technology.
In addition, the frequency of an optical signal is usually as high as 200 THz, so that there often occurs frequency shift of several GHz. Anyway, such large frequency offset is disadvantageous to the coherent system, even in the case of using the phase locked loop.